Defensively, he is very strong. Physically he can bully lesser guards. And he has enough rep that the refs let him play D without him fouling out of the game. It's hard to find this type of package in a 6'4 point guard.
Offensively, still a work in progress, but his ability to take long range shots definitely helps with the floor spacing. The Defense has to honor his 3-ball. Something that a certain other former PG on this team could not bring to the table. Just by being a credible shooting threat, Smart allows other players more room to operate on offense.
He's not a primary ballhandler yet, but I'm sure he will learn over time how to run the offense. With creative players like Evan Turner and Isaiah Thomas available, Brad Stevens doesn't have to turn the keys over to Smart yet until he is fully ready.